A galvanically isolated voltage converter is an electronic system that has a primary side and a secondary side coupled electrically by a transformer. The primary side of converters comprises a controller circuit and a switching power transistor and the output terminal of the power transistor is coupled to the primary winding of the transformer. The secondary side comprises a rectifying device coupled to an output capacitor and to the secondary winding of the transformer. During normal operation the capacitor maintains an output voltage near a specified value.
The rectifying device may be a p-n junction diode or a Schottky diode, which is an uni-directional device configured to conduct current during the periods when the switching transistor in the primary side is switched off and blocks current flow during the periods when the switching transistor in turned on. This type of rectification is referred to as non-synchronous rectification.
An alternative to a diode for rectification is a power MOSFET, which is a bi-directional device and is configured to turn on to conduct current and to turn off to block current flow following a command signal from a controller circuit. The controller on the secondary side of the converter may be a single integrated circuit chip, which may or may not encompass the MOSFET. A well-arranged combination of a secondary side controller and a MOSFET dissipates less power than the rectifying diode it replaces. This type of rectification is referred to as synchronous rectification.
At the occurrence of abnormal output voltage drop, the converter shuts down to protect the system from damage by activating under voltage lock out (UVLO) circuitry on both sides of the converter. That is, the primary side stops pumping energy into the secondary side and the secondary side stops charging the output capacitor.